It is known, in general, to produce ceramic precursor powders by precipitation of such a precursor from a homogeneous solution containing the desired cations as generally indicated in U.S. Pat. No. 4,999,323 at Col. 1, lines 5-50. However, as indicated in that patent, the products can be aggregated powders which can require further treatments such as milling and/or washing with "organic solvents". Of similar teaching is U.S. Pat. No. 4,501,818 at Col. 1, lines 17-38 and U.S. Pat. No. 5,008,092 at Col. 1, line 61 to Col. 2, line 3. One example of a literature reference in this area, which describes the fabrication of alumina-zirconia compositions from an aluminum chloride/zirconium chloride solution by the addition of ammonium hydroxide as the basic precipitation agent at a pH of up to 9, is described in Ceramic Bulletin, Vol. 68, No. 5, 1989, pp. 1017-1020.
It has been found by the present inventors that the addition of such aqueous solutions containing ceramic precursor metal cations to a basic precipitation agent can produce gelation, especially if very dilute solutions of the cations (such as employed in U.S. Pat. No. 4,810,680) are not used. For example, when yttria-stabilized zirconia is the intended product and an aqueous solution is formulated containing a predominant (e.g., about 92%) of a zirconium salt and a much lower amount (e.g., about 8%) of a yttrium salt, gelation problems can occur between pH readings of about 6 and 9 if relatively concentrated solutions are used (e.g., about 15 gm to 40 gm of metal cation (expressed as oxide per 100 ml water).
One approach to solving such gelation problems is advocated in the aforementioned U.S. Pat. No. 4,501,818 which advocates the use of a "substantially anhydrous" reaction medium with the solution of ceramic metal precursor compounds being added to the base rather than the converse. Water has been implicated in the formation of "hard agglomerates" in the washing of certain ceramic powders (i.e., zirconia) formed by the sol-emulsion-gel method (see J. Am. Ceram. Soc., 73(9) 2760-2763 (1990)).